In general, an image sensor is a semiconductor device that transforms an optical image to electrical signals. The image sensor is generally classified as a charge coupled device (CCD) or a CMOS image sensor.
The CCD includes a matrix of photodiodes (PD). Each photodiode converts an optical signal into an electric signal. The CCD also includes a plurality of vertical charge coupled devices (VCCDs). The VCCDs are formed between vertical lines of the photodiodes in the matrix for transmission of charges generated at the photodiodes in a vertical direction. The CCD further includes horizontal charge coupled devices (HCCDs) for transmission of the charges transmitted through the VCCDs in a horizontal direction. In addition, the CCD includes a sense amplifier for sensing the charges transmitted in the horizontal direction and outputting an electric signal.
However, the CCD is disadvantageous in that it has a complicated driving method, exhibits high power consumption, and is produced via a complicated fabrication process involving multiple photo process stages.
Moreover, the CCD has another disadvantage in that it is difficult to include a CCD in a small product due to the difficulty in integrating a control circuit, a signal processing circuit, and an A/D converter, and the like on a CCD chip.
Recently, the CMOS sensor has been heralded as the next generation image sensor that can overcome the disadvantages of CCDs.
The CMOS image sensor is a device that employs CMOS technology to capture an image. Specifically, a control circuit, a signal processing circuit, and the like are used as peripheral circuits for successively detecting outputs from pixels using MOS transistors. A MOS transistor is formed on the semiconductor substrate for each pixel.
That is, the CMOS image sensor has a photodiode and a MOS transistor formed within each unit pixel. By monitoring the switching of the MOS transistors, the CMOS image sensor successively detects electric signals from the photodiodes of the unit pixels to reproduce an image.
The CMOS image sensor exhibits low power consumption and has a simple fabrication process as a result of fewer photo process stages.
The following is a description of a method for fabricating a CMOS image sensor according to the related art with reference to the accompanying drawings.
FIGS. 1A and 1B are cross-sectional views of a CMOS image sensor for describing a method of fabricating a CMOS image sensor according to the related art.
Referring to FIG. 1A, an interlayer dielectric 12 is formed on a semiconductor substrate (not shown). Here, the substrate includes a plurality of photo sensing devices, for example, photodiodes 11.
The interlayer dielectric 12 may be formed with multi layers. Although not shown, after one interlayer dielectric layer has been formed, a light shielding layer may be formed and another interlayer dielectric layer can then be formed thereon. Here, the light shielding layer functions to block light incident on parts of the substrate other than photodiode regions.
Next, a planarized passivation layer 13 is formed on the interlayer dielectric 12 in order to protect a device from moisture and to prevent the device from being scratched.
Then, color filters 14, which filter light by wavelengths, are formed by coating the passivation layer 13 with a dyable resist, and then performing exposure and developing processes.
Here, a photolithography process is selectively performed in the red (R), green (G), and blue (B) color filter layers 14 three times to form a color separating layer.
Next, a planarization layer 15 is formed on the color filter layers 14 in order to adjust a focusing distance and secure the flatness for forming a lens layer.
Further, a thermal treatment is performed at a temperature greater than 200° C. so as to cure the planarization layer 15.
Then, a resist layer for a microlens is coated on the planarization layer 35, and a reticle (not shown) having an opening portion is aligned above the resist layer. Next, light such as a laser beam is irradiated to an entire surface of the semiconductor substrate using the reticle as a mask to selectively expose the resist layer through the openings of the reticle.
Then, the exposed resist layer 16a is developed to form a microlens pattern 20.
Thereafter, as shown in FIG. 2B, the microlens patterns 20 are reflown at a predetermined temperature to form microlenses 22.
However, during the forming process of the microlens pattern 20 it is difficult to precisely define intervals between microlens patterns because of the limit of the resolution of photolithographic equipment. Moreover, after completion of the reflow process, adjacent microlenses will not have a zero-gap.
Consequently, some light may not be focused to a light receiving section of the CMOS image sensor, which results in the deterioration in the sensitivity of the CMOS image sensor.